Inspection method and means



United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE The liquid dye penetrant consisting of a fast-drying, volatile, low viscosity solvent and a bleeding dye soluble therein is used for deep penetration into minute surface defects. The viscosity range of the solvent is from about .05 to 2.00 centipoises at room temperature.

This is a continuation-in-part of application Ser. No. 253,254, filed Jan. 23, 1963, now U.S. Patent No. 3,279,243, and copending therewith.

This invention relates to inspection and testing of specimens of various materials and surface conditions to locate and identify discontinuities, surface textures, lines of incipient failure, voids, cracks, or the like. More particularly, the invention contemplates a process and a new combination of materials resulting in improved economy and versatility for inspection or testing of specimens by dye penetrants whereby a clear, precise, permanent reproducible record of the inspection results is rapidly obtained.

Penetrants known in the prior art include various liquids mixed with dye or pigment, and are often used in combination with a so-called developer or bottling agent. Use of a liquid penetrant known to the prior art in a typical case may involve application of the penetrant to the specimen followed by a soaking period such as thirty minutes or longer to permit penetration of the liquid into the specimen material. Following the soaking period, removal of excess penetrant by appropriate cleaning is accomplished, usually involving a suitable solvent to aid such removal. Depending upon the penetrant composition, an emulsifying agent is sometimes used to transform excess penetrant as necessary for solubility of the penetrant in a cleaning solution. After cleaning, the specimen surface may be air dried for an additional period of time to dry the specimen surface but not the liquid penetrant remaining in the surface voids or cracks, after which the specimen may then be coated with the developer or blotting agent. Various powders such as ground silica, powdered chalk, or talcum are commonly used as developers, and function to provide a relatively light or otherwise contrasting background whereby the dye will more clearly appear. More importantly, the developer functions to blot residual traces of the liquid dye out of surface voids or cracks in the specimen surface, usually by a process of absorption. The pattern of blots thus produced by the liquid dye on the developer shows the general location and shape of the stated surface discrepancy.

The typical process described above, and minor variations thereof, are widely used in industry. Many dye penetrant inspection procedures currently known require considerable time to complete the inspection. In addition to 3,433,062 Patented Mar. 18, 1969 their time-consuming aspects, conventional processes have the further disadvantage that the blotting action of the developer depends upon absorption of the liquid dye penetrant by the powder-like particles of the developer. Since this absorbing action continues as long as liquid dye remains in contact with the developer, the pattern initially produced quickly becomes fuzzy and is eventually obliterated so that definition is lost in a matter of minutes. Moreover, the definition obtained in the dye pattern depends partly upon grain size in the powder constituent of the developer whereby larger grain sizes produce poorer definition, and even the smallest grain size compromises the clarity of the pattern, as evidenced by the fact that the blot pattern is always of different size and shape than the void or defects indicated by the blot.

A further drawback of the conventional process is that the developer, especially where powdered materials are used for this purpose, normally wipes olf immediately upon light touching so that specimens must be handled with utmost care. Also, conventional penetrants are of little or no use in detecting flaws in ceramic and porous materials, either metallic or non-metallic. One reason for their failure in this regard is that conventional penetrants produce a concentration of liquid in voids, and it is this action upon which the usefulness of most penetrants known to the prior art depends in determining the location of surface voids. The blots produced by such concentrations are always magnified, that is, characteristically larger in size than the void or defect itself. Thus, two voids which are in close proximity will produce a single blot whereby they cannot be separately detected from close scrutiny of the dye pattern. Porous materials such as scintered metallic parts contain many voids which may be extremely close together and which are normal and acceptable features of porosity, but the mass of blots produced by conventional penetrants as a result of the normal voids entirely obscures or obliterates all indication of any deleterious voids or structural cracks which may exist in porous specimens. Moreover, known liquid dye penetrants are severely limited regarding temperatures at which they may be used. In general, known penetrants begin losing their penetrating power at temperatures below F., primarily due to change of surface tension and flow characteristics, and some dye penetrants gradually thicken in consistency, becoming paste-like at temperatures of 50 F. or below.

Accordingly, it is a principal object of this invention to provide improved penetrant and developer materials capable of providing rapid inspection results having great clearity, definition, and permanency.

Another object in this case is to provide a dye penetrant having improved penetration power for use in a wide variety of metallic and non-metallic materials, both porous and non-porous.

Another object in this case is to provide a rapid drying liquid penetrant which requires no special drying procedures.

It is another object in this case to provide an improved developer for use in dye penetrant inspections.

Another object in this case is to provide a developer as stated in the preceding object capable of providing a permanent record of the inspection results.

It is a further object in this case to provide a dye penetrant and developer combination as set forth in the above objects operable over a wide range of specimen and environmental temperatures, especially sub-freezing temperatures.

It is a further object of this invention to provide an improved developer as set forth in the above objects capable of rapidly producing a pattern and which achieves and maintains improved clarity and definition in such pattern over long periods of time.

Another object in this case is to provide improved penetrant inspection test results as described in the above objects capable of being removed from the specimen and filed or reproduced by normal photographic processes.

Other objects and advantages of the invention in this case will become apparent upon a close reading of the following detailed description, including an illustrative embodiment of the inventive concept.

The invention disclosed herein provides a method for non-destructive inspection of specimen surface discrepancies such as minute cracks, voids, pits or the like, comprising the steps of applying a quick-drying penetrant to the surface, removing excess penetrant from the surface, and finally applying to the specimen surface a relatively quick-drying developer in which the dye constituent of the penetrant is adapted to bleed only until the developer becomes substantially dry. Thus, the dye penetrant used in practicing the novel method in this case includes a quick-drying solvent which serves primarily as a carrier for the dye. In general, this results in more rapid and effective penetration of the dye in surface voids than that achieved by relatively slow evaporating solvents and which require prolonged soaking periods. Moreover, the dye which is deposited within minute cracks and voids in the specimen surface, quickly becomes dry after application of the penetrant so that application of the liquid developer is not accompanied by excessive or uncontrollable bleeding. The dry dye, upon contact with the developer, migrates briefly through the developer coating until the coating becomes dry. A printed pattern is thus produced by the dye on the developer characterized by extreme clarity, detail and definition exactly reproducing each minute crack or void in the specimen surface without magnification in the size of each detail of the pattern compared with the size of the defect so indicated. Both the dye and developer materials provide the results described above even though the specimen and environmental temperature be in the sub-freezing range as much as 50 F. or more below zero.

Now considering the details of the inventive concept, the dye constituent of the penetrant used in this case is required to be of a type which is soluble in the solvent used in the developer, and which is capable of bleeding or migrating through the developer only until such solvent evaporates or such developer becomes substantially dry. It is also important that the dye be capable of such bleeding upon contact with the moist or liquid developer when said dye is in the dry or substantially dry state. It is another necessary feature of the dye that the pattern produced by its migration has only slight magnification, and that the amount of migration is arrested and the pattern thereby fixed rapidly after initial forming by drying of the developer. In this context, the term fixed is used to refer to a physical action similar but not limited to the phenomenon of adsorption. Thus, controlled bleeding of the dye is necessary to produce maximum clarity and definition of the inspection pattern.

With further regard to the dye, the inventive process disclosed herein may be adapted for use with any oil or spirit soluble bleeding dye having the necessary properties discussed above, especially insoluble azo type dyes; i.e., characterized by N-to-N bonds. The insoluble azo type dyes contain no water-solubilizing groups. A dye known by various names in trade such as Oil Red and Sudan Red sold by the General Aniline and Film Corporation is illustrative. The stated dye has the general formula xylene azo beta napthol and the empirical composition C H N O. Mefford No. 322 dye comprising O-toluene azo xylene azo beta napthol is further illustrative. Also, fluorescent dyes are useful such as Fluorol 7GA sold by General Aniline and Film Corporation. In addition, xanthene dyes such as Rhodamine B extra dyes have been found very satisfactory, especially in connection with specimens of glass or ceramic. Rhodamine B dye is sold by the General Aniline and Film Corporation and has the formula C H N O Cl. It is variously identified as a hydroxide of diethyl-mono-amino phenol-phthalein, and as tetraethyl diamino-ortho-carboxy-phenyl-xanthene chloride. Rhodamine dyes include a broad class containing some which are water soluble, and which work very successfully in the penetrant inspection method disclosed herein, especially when a wetting agent is added to the liquid dye penetrant such as an aerosol as mentioned herein-below.

With specific regard to characteristics of the penetrant contemplated in the instant case it is of importance that the penetrant be capable of drying substantially completely and very rapidly after being applied to a specimen surface without necessity for elaborate drying procedure. Thus, the solvent used in the dye penetrant functions only as a carrier whereby the penetrant flows freely over a specimen surface and conveys dye into cracks, voids, or the like on such surfaces, and thereafter the penetrant quickly dries by evaporation of the solvent. Due to the desirability of rapid drying, primarily by evaporation of one or more consituent elements in the dye penetrant, use of slow drying solvents, particularly oily solvents, is avoided. In addition to the solvent or carrier used in combination with the dye to provide a penerant such as discussed above, the addition of a wetting agent such as an aerosol to the dye penetrant is optionally useful to render it alcohol soluble for later cleaning purposes as discussed in greater detail hereinbelow. The addition of the aerosol constituent is not essential in achieving advantageous results by the methods and compositions taught herein, but is useful especially where maximum penetration is porous materials or particularly minute defects is desired.

The penetrant or penetrant and developer combinations disclosed herein are adaptable for use with the common organic solvents known to the art having the above discussed properties. Thus, in the case of an oil soluble system, both the penetrant and the developer solvents may be selected from the same group within the broad class of oil type solvents which may be aromatic, aliphatic, or chlorinated. Such solvents should be of the type having relatively low molecular weight for the purpose of rapid drying or evaporating. Examples of suitable aliphatic solvents usable both in the dye penetrant and developer are ketones such as methyl ethyl ketone, methyl isobutyl ketone, xylene and acetone. Examples of suitable aromatic solvents usable in the dye penetrant and developer are toluene and benzene. Examples of suitable chlorinated solvents usable in the stated context are methylene chloride, trichlor ethylene, and carbon tetrachloride. Regarding adaptation of the novel method disclosed herein for use with a spirit soluble system, various alcohols are suitable solvents for both the dye penetrant and the developer, such as methenol, denatured ethanol and isopropyl alcohol. Regarding the proportions of mixture for the dye and solvent or vehicle in combination, acceptable results have been obtained using one part dye in 50 parts of the solvent by volume. Also, one part dye in 7 parts of vehicle is a useful ratio, but any less proportion of the solvent results in a paste-like consistency which inhibits the Penetration of the dye in specimen surface defects. However, optimum results are obtained using one part dye in 14 parts of vehicle by volume. The heavier concentration of dye in a one to 14 mixture permits repetition of an inspection many times without the necessity of recoating the specimen surfaces with additional dye penetrant. Thus, after one application of dye, a developer coating may be succissively applied many times and removed each time without redying the same area.

Now considering the developer used in practicing the novel method disclosed herein, it is distinctl significant that the present developer does not blot or absorb the dye penetrant as generally associated with developers previously known in the art and which usually require the dye penetrant to be in the liquid state when the developer is applied to the specimen surface. Particularly advantageous in practicing the method disclosed herein are developers which include one or more resins as a binder in combination with a solvent selected from the classes discussed above. In such developers, flexibility of the binder is achieved by a suitable plasticizer. It is the principle function of the developer to provide a precise and permanent representation of the discontinuities in the specimen surface with a resolution not obtainable by ordinary examination. The developer of the present invention controls, among other things, the amount of rate of bleeding caused by the dye upon initial contact with the developer and arrests the bleeding action completely after a brief period. While various resins are usable in the developers discussed above, namely, vinyls, acrylics, nitrocellulose, butrates, and latex, the vinyl polymers and copolymers have been found to produce results superior to all other resins. For example, polyvinyl chloride resin and polyvinyl chloride-polyvinyl acetate copolymers have been found particularly advantageous. The developer may be clear lacquer such as the type discussed in greater detail below or else may have a color contrasting with the dye constituent of the penetrant. The plasticizer governs the rate of evaporation of the volatile solvents out of the developer and determines the strippable properties of the resulting coating. Among suitable plasticizers for use in combination with various of the resins discussed above are diisoctyl phthalate, dioctyl phthalate, butyl phthalate, and tri-Z-ethylhexyl. Other commercially available plasticizers known to the art may be used with the above plastics.

In addition to developers having a resin constituent, other fast drying developers have been successfully used with the dyes discussed above, some being non-strippable however, and others involving less satisfactory results in various respects. The developer is preferably in liquid form, is quick-drying primarily due to rapid evaporation of the volatile solvents therefrom, and does not require special drying procedures. The pattern produced by the dye on the developer and which becomes perceptible within a few seconds after its application to the specimen thus quickly solidifies or otherwise becomes fixed and chemically stable after which no further significant change in such pattern occurs after the developer has dried. Inspection of the specimen upon which the process discussed above is practiced may be accomplished without removing the pattern from the specimen. However, if a strippable paint or lacquer is used in the developer, the inspection pattern may be stripped or peeled from the surface of the specimen in one or more continuous strips which may thereafter be stored, filed, or else used as photographic negatives to produce prints on sensitized paper by normal photographic methods, including enlargement to facilitate study of the inspection pattern. Where developers are capable of being sprayed, electrostatic spraying of the same is useful in avoiding large dip tanks, because spraying one side of a specimen provides a coating over all surfaces of the specimen.

In further connection with the novel method disclosed herein, it has been found that some plastic vinyl tapes, such as those in common surgical use, may be applied to a treated specimen surface in dry form and will act as a developer coating when activated by moistening with a solvent of the type discussed above, such as methyl isobutyl ketone. However, while such tapes provide some advantages over the prior art, they are not as rapid or effective generally as the rapid drying liquid developers dicussed above.

The examples given below are illustrative of the benefits and advantages obtained from use of the novel teachmgs contained herein and are representative only.

EXAMPLE I The inventive method disclosed herein was practiced with great success in disclosing extremely minute surface discrepancies on metallic specimens of advanced alloy stainless steel and fusion welded joints on such specimens. A penetrant solution comprising 14 parts methyl isobutyl ketone, 1 part of xylene azo beta napthol and 2 parts aerosol by volume was prepared. The dye penetrant was applied to weld seams and workpiece surface portions by brushing, and required no soaking period to achieve effective coverage or penetration. The methyl isobutyl ketone evaporated very quickly after carrying the dye into surface cracks and then was not a significant factor in the remaining steps of the process. The dyed portion was then superficially cleaned by a solution of equal parts methyl alcohol and isobutyl ketone which also quickly evaporated with no significant residual effects on the inspection process. Thereafter, the specimen surface was spray coated with a developer which included a vinyl base paint comprising about 19% polyvinyl chloride copolymer resin, 1% white mineral oil, about 61% toluene, 14% methyl ethyl ketone, 6% diisoctyl phthalate, and a nominal amount of non-chalking rut tile titanium dioxide sufiicient to give the necessary amount of whiteness for contrast with the red dye. A thinner of methyl isobutyl ketone was added to the foregoing paint to reduce the same to spraying consistency. The sprayed coating immediately picked up dye traces, some of which were barely visible without microscopic aid, after which the developer coating quickly dried and the dye pattern formed thereon became permanently fixed and unchangeable. The pattern was characterized by a virtually complete absence of magnification in respect to the actual size of the surface void or crack compared with the trace produced in the final pattern on the developer coating.

The resulting dried developer was then peeled from the specimen surface in one continuous layer by manually pulling on one end and later was used as a photographic negative for printing on sensitized paper.

EXAMPLE II The novel method disclosed herein was successfully practiced in a water soluble system using a Rhodamine dye (sold under the trade name Rhodamine B extra S by General Aniline and Film Corporation) mixed with isobutyl ketone. The dye penetrant was applied to a stainless steel specimen by brushing, followed by surface cleaning as discusssed in Example I above. Thereafter, a developer using a water soluble system and comprising a mixture of ethyl cellulose and titanium dioxide in a commercial product known as Logo, sold by the Bee Chemical Company to which a vehicle of dentatured alcohol was added, was applied to the inspection area by spraying. The resultant dye pattern formed quickly and became permanently fixed upon drying of the developer. The pattern was of superior clarity and permanency in comparison with conventional dye inspection results; however, the degree of definition was slightly less than that associated with the results from Example I. The dye pattern and developer coating were subsequently removed from the specimen by water washing.

EXAMPLE III A fluorescent type penetrant was used in combination with a developer of transparent lacquer to inspect weld seams in advanced alloy stainless steel specimens. Fluorescent dye in the form of Fluorol 7GA was combined with methyl ethyl ketone which constituted the vehicle or carrier for the stated dye. After superficial cleaning of the specimen surfaces in the manner described for Example I above, a coating of liquid developer having a composition identical to that specified in Example I above except omitting the titanium dioxide coloring agent, was sprayed upon the specimen surface. Omission of the pigment from the stated developer results in a clear vinyl lacquer upon which a fluorescent pattern was formed by the static dye. After the developer coating had dried, which occurred very quickly, the developer coating with the pattern printed thereon was peeled from the specimen surface in one continuous strip, and used for printing one photographic paper. Also, a very clear and well defined pattern was visible from the stripped coating when viewed under ultraviolet light.

The novel methods disclosed herein using the materials illustratively mentioned or otherwise discussed above have been found to provide extremely rapid and most effective inspection techniques. Thus, quenching cracks which are normally very minute, and other surface conditions appear clearly and quickly within a period of one minute and often within five or ten seconds after application of the developer on a treated specimen. Due to the characterstics of the developer and the dye penetrant, especially with respect to the quick-drying characteristics of both, the pattern initially produced by the dye is arrested before any significant magnification occurs, and is permanently fixed whereby no change in the pattern occurs although the inspection results be stored or otherwise remain unattended for hours, weeks or months, whether removed from the specimen surface or not.

The present inspection 'method and combination of materials may be practiced with diverse workpiece or specimen materials, and has been found to provide extremely accurate, detailed and rapid inspection results in connection with porous materials, wood, plasma sprayed metals, glass, ceramic, and many others, including ice.

Other examples and results from actual tests illustrating the inventive concept discussed above are set forth below. In all examples, the dye penetrant solution contained the same ratio of one part dye, 14 parts vehicle, and 2 parts of an aerosol by volume. The aerosol was 75% pure, and was included mainly as a wetting agent and to facilitate cleaning with alcohol following the test. Also, in each of the following examples, the developer and vehicle were mixed in a ratio of 2 parts vehicle to 1 part of developer, with slight variations as necessary to reduce the developer to spraying consistency. All the tests involved application of the dye by brushing upon the specimen surface and spraying of the developer and vehicle combination, and included specimen materials of steel. wood, glass and various plastics as well as those specified below.

EXAMPLE IV A dye penetrant mixture of Oil Red and a vehicle of methyl isobutyl ketone was applied to metallic specimens comprising fusion welded joints in steel and specimen layers of plasma sprayed aluminum dioxide over steel. The excess dye was cleaned from the specimen in each case in the manner described for Example 1 above. A coating of strippable developer comprising a mixture of methyl isobutyl ketone and vinyl base paint sold commercially as Brolite by the Andrew Brown Paint Company was then sprayed on specimen surfaces. Brolite has a composition generally similar to that described for the vinyl base paint in Example I above. A dye pattern showing surface defects developed immediately after the developer coating was applied and defects were extremely well defined by dye traces in the pattern. The same combination of dye, developer and vehicle was used with similarly successful results on specimens of ice and of precipitation hardenable stainless steel while both the dye and the steel were at a temperature of 50 F. below zero.

EXAMPLE V A dye of Rhodamine B extra S combined with a vehicle of ethyl isobutyl ketone was applied to metallic specimens, the excess dye removed, and a developer similar to Example IV above thereafter applied. Excellent delineation of surface defects was achieved in the resulting dye pattern, and the developer coating was peeled from the specimen in one continuous strip. This combination of materials was also found to provide consistently excellent results at specimen and environmental temperatures 50 below zero. The Rhodamine dye used in this test is daylight visible as well as under black light, i.e., ultraviolet light or near the ultraviolet range.

EXAMPLE VI A dye penetrant comprising Fluorol 76A and methyl isobutyl ketone was applied to metallic specimens and the excess dye thereafter removed. A developer identical to that used for Example IV and V described above except omitting the titanium dioxide pigment, was thereafter applied by spraying. Since the dye used in this test was visible in ultraviolet light or light near the ultraviolet range, use of a clear developer permits greater visibility of the dye trace. Excellent delineation of the inspection pattern was obtained, and the strippable developer coating was peeled from the specimen in one continuous strip for detail and study under black light and for making photographic prints therefrom. The dye and developer combinations in this example were also found to produce consistently superior results with specimen and environmental temperatures in the sub-zero range, such as 50 F. below zero.

EXAMPLE VII A dye similar to that used for Example IV above was applied to metallic specimens and the excess removed therefrom. Thereafter, a developer coating comprising nitrocellulose lacquer complying with military specification MIL-L-7l78 and a vehicle of methyl isobutyl ketone was applied by spraying. The dye trace developed immediately upon application of the coating and delineation of the dye trace was excellent. The developer dried rapidly and was non-strippable from the specimen surface.

EXAMPLE VIII The dye used in Example IV above was applied to metallic specimens, the excess dye removed, and a developer comprising cellulose nitrate dope and a vehicle of methyl isobutyl ketone was applied by spraying. The resulting dye trace appeared less rapidly than occurred in Examples IV through VII discussed above, and the color of the dye changed from brilliant red to a purple hue less sharply visible than that characterizing the test results discussed above. The final test pattern was not strippable from the specimen.

EXAMPLE IX A dye penetrant identical with that used in Example IV above was used in combination with a developer comprising butyrate cellulose acetate dope and a suitable thinner. The pattern was less visible and less clearly defined than that obtained from the materials described in Examples 1V through VII above.

EXAMPLE X A dye coinciding with that described above in connection with Example IV was used in combination with a developer comprising chlorinated rubber enamel paint with a vehicle comprising xylene. Following application of the developer and vehicle, a dye trace indication appeared very slowly and of less visible color than that characterizing the dye itself before the developer coating was applied. However, the dye pattern had very good definition. The inspection results were not strippable from the specimen due to inherent characteristics of the enamel.

EXAMPLE XI A dye corresponding with that described in connection with Example IV was used in combination with a developer and vehicle comprising alkyd resin sold commercially as Kern Lustral Enamel by the Sherwin-Williams Paint Company, in combination with a vehicle of naphtha. The resulting dye pattern was superior to that obtained in connection with Example X above but inferior with respect to clarity and contrast of the patterns obtained from Examples IV, V, and VI above. The inspection pattern was non-Strippable.

EXAMPLE XII A dye corresponding to that described for Example IV above was used in combination with the developer described in connection with Example II above on metal surfaces. The resulting inspection pattern, although characterized by definition superior to that obtainable from conventional dye penetrants, was poorer than that obtained in connection with Examples IV through XI above. The developer coating was non-strippable.

EXAMPLE XIII A dye penetrant comprising one part of Oil Red 10 parts of methyl isobutyl ketone, and parts of polyvinyl chloride base paint corresponding to that described for the developer in connection with Example I above except omitting titanium dioxide pigment, was prepared. The foregoing dye penetrant Was applied with brush to various porous materials including plasma sprayed aluminum dioxide, scintered metals, and the like, and allowed to dry. A rinse solution of three parts of isopropyl alcohol and one part methyl isobutyl ketone was quickly brushed on the dried area, after which the coating formed by the dye on the specimen surface was peeled off in one continuous strip. Thereafter, a developer and vehicle combination identical to that described in connection with Example I above was spray coated over the area previously covered by the dye, whereby a pattern of dye traces rapidly appeared in the developer coating and became fixed. The dried developer coating was peeled off in one continuous strip.

EXAMPLE XIV The test materials and procedures described in connection with Example XIII above were repeated except for the substitution of the fluorescent dyes used for Examples V and VI above in place of Oil Red 0. The inspection results were in each case equally as satisfactory as those obtained from Example XIII above. As in the case of materials described in connection with Example XIII above, the materials tested in this example were found to produce consistently excellent dye patterns when specimen and environmental temperatures were as low as 50 F. below zero throughout the temperature range to a maximum of 110 F. above zero.

It is of particular significance in connection with the inventive principles disclosed hereinabove that the dye penetrant compositions using low viscosity dye solvents illustratively specified above as the vehicle for the dye achieve rapid, deep and complete penetration of dye into minute surface cracks and voids. Thus, the various penetrant solvents selected from the oil type solvents disclosed herein, whether aromatic, aliphatic or chlorinated, are not essentially of oily nature but are oil solvents, and are characterized by viscosities significantly less than those identified with oils. Most of the mentioned embodiments for both oil soluble and spirit soluble systems have viscosities of a relatively low order such as within the range from about .100 to 2.00 centipoises at room temperature (about 70 F.). A maximum of about five centipoises for the penetrant vehicle is workable in revealing large defects, but solvents having the lesser viscosity values from .100 to .600 centipoise at 70 F. provide advantageous and more versatile results in respect of smaller and deeper surface cracks or voids than solvents identified with the higher limit of the stated range. Low viscosity solvents used as the entire or sole vehicle in a dye penetrant provides the stated advantages due primarily to the increased penetrating power and avoidance of meniscus etfects in such solvents, whereby a minute crack or void can fill completely with penetrant rather than form a shallow meniscus bridge of liquid between opposite walls thereof as commonly results from relatively high viscosity oils. Light machine oil as used in some liquid dye penetrants as a vehicle known to the prior art has a typical viscosity of about 34 centipoise even at elevated temperatures such as F. Moreover, the inspection process using dye penetrant compositions disclosed herein is considerably accelerated by avoidance of the need to soak the specimen or immerse the same completely for protracted periods of time as necessary for oily dye penetrants well known to the prior art. The dye vehicles disclosed herein achieve complete and rapid penetration in porous specimens of diverse materials, whereby a few drops of the penetrant when applied to a wood surface, for example, immediately spread throughout the specimen mass almost at the very moment of contact. The foregoing advantages and results may be seen to relate to liquid dye penetrant compositions disclosed herein even without regard to the presence or absence of a developing agent, and may sufiice for useful inspection results in some cases whereby sufiicient contrast of the dye with the specimen material inherently results from the color thereof without the addition of activating or of contrasting agents.

While the particular details set forth above are fully capable of attaining the objects and providing the advantages herein stated, the specific materials and methods thus disclosed are merely illustrative and could be varied to produce the same results without departing from the scope of the inventive concept as defined in the appended claims.

I claim:

1. A dye penetrant for protecting surface cracks and the like in specimens, said penetrant consisting of:

a quick-drying volatile solvent, and a bleeding type dye soluble in said solvent added thereto in an amount sufficient to provide dry residual dye remaining within said surface cracks with a visible dye stain in contrast with said specimen surface, said amount being within a range from about 1:7 to 1:50 parts dye to solvent, by volume.

2. A dye penetrant as set forth in claim 1 above, wheresaid solvent comprises methyl ethyl ketone, and said dye comprises an insoluble azo type dye.

3. A dye penetrant for detecting surface defects and the like in specimens, said penetrant consisting of:

a quick-drying low molecular weight solvent, and a dye comprising xylene azo beta napthol, said dye being within a range from about 1:7 to 1:50 parts by volume of said solvent.

4. A non-exuding dye penetrant for detecting surface defects and the like in specimens, said penetrant consisting of:

a. low viscosity solvent and a bleeding type dye, said solvent having a viscosity within the range from about .100 to 2.00 centipoises at a temperature of 70 F., and said dye being soluble in said solvent, said dye being in a range from about 1:7 to 1:50 parts by volume of said solvent.

5. A liquid dye penetrant for detecting surface cracks and the like in specimens, said penetrant consisting of:

a low viscosity solvent and an insoluble azo type dye soluble in said solvent, said solvent being selected from a group consisting of aromatic, aliphatic or chlorinated type oil solvents and having a viscosity within the range from .050 to 2.00 centipoises at a temperature of 70 F., said dye being within a range from about 1:7 to 1:50 parts by volume of said solvent.

6. A method of inspection for surface discontinuities in a specimen, said method comprising:

applying to said specimen surface a relatively quickdrying liquid penetrant consisting of a low viscosity solvent and a bleeding type dye soluble in said 11 solvent, said solvent having a viscosity within the range from .050 to 2.00 centipoises at a temperature of 70 F., said dye being within a range from about 1:7 to 1:50 parts by volume of said solvent,

ketone and an insoluble azo type dye added thereto in an amount sufficient to provide a visible dye stain on said specimen surface, said dye "being from about 1:7 to 1:50 parts by volume of said solvent.

and cleaning said surface to remove excess penetrant 5 therefrom, leaving substantially dry dye in said discontinuities.

References Cited UNITED STATES PATENTS 7. A method of inspection for surface discontinuities 2,340,940 2/ 1944 De Forest 7351 in a specimen, said method comprising: 2,478,951 8/ 1949 Stokely 252408 applying to said specimen surface a quick-drying liquid 10 2,871,697 2/ 1959 Sockrnan 73-104 dye penetrant consisting of a volatile solvent, and a bleeding type dye soluble in said solvent added thereto in an amount sufficient to provide a visible dye stain on said specimen surface, said dye being from 15 RICHARD C. QUEISSER Primary Examiner about 1:7 to 1:50 parts by volume of said solvent. 8. A method of inspection for surface discontinuities IRVIN MCCLELLAND Ass'stant Exammer' US. Cl. X.R.

in a specimen, said method comprising:

applying to said specimen surface a relatively quickdrying liquid penetrant consisting of methyl ethyl 20 252 307 328 127 FOREIGN PATENTS 709,101 5/1954 Great Britain. 

